1. Field of the Invention
The present invention relates to an exhaust pipe for an internal combustion engine, the exhaust pipe being used for guiding along an exhaust gas which is generated in the internal combustion engine. More particularly, the present invention relates to an exhaust pipe for an internal combustion engine such that at least a portion of the exhaust pipe appears on the exterior of a vehicle which incorporates the internal combustion engine.
2. Description of the Related Art
In many motorcycles or the like incorporating an internal combustion engine, the internal combustion engine is exposed in such a manner that the exterior appearance of the internal combustion engine can contribute to the aesthetic design of the motorcycle. In such a motorcycle, an exhaust pipe for guiding along exhaust gas from the internal combustion engine also plays an important role in the design of the motorcycle. Even in the case where the internal combustion engine is covered by a cowl or the like, the exhaust pipe is not likely to be completely covered by a cowl or a protector. Rather, it is often the case that at least a portion of the exhaust pipe appears on the exterior to become a part of the motorcycle design.
FIG. 11 is a side view showing an example of a sports-type motorcycle. A motorcycle 200 shown in FIG. 11 includes a V-type engine 201 and an exhaust pipe 202 for guiding along exhaust gas. Two cylinders of the V-type engine 201 become united into the single exhaust pipe 202, which extends toward and above the rear wheel so as to allow exhaust gas to be discharged at the rear portion of the body. In the present specification, an “exhaust pipe” means the entirety of any structure which composes a channel for guiding along the exhaust gas from the internal combustion engine, and may include any portion composing a muffler 202a or the like.
In general, the exhaust pipe 202 must have a certain thickness for allowing the exhaust gas generated in the internal combustion engine 201 to be efficiently discharged. Moreover, the portion constituting the muffler 202a has an increased diameter in order to accommodate the muffling structure. For this reason, the exhaust pipe accounts for a relatively large part of the exterior appearance of the entire motorcycle, and thus, the shape and color of the exhaust pipe have an impact on the entire motorcycle design.
For these reasons, the shape and color of the exhaust pipe are important factors in deciding the entire motorcycle design. Therefore, it is common to construct the exhaust pipe in a smoothly bent shape to present a strong and voluminous impression, or finish the surface of the exhaust pipe with a lustered metal color or finish for an enhanced contrast against the other component parts, for example. Conversely, it is also common to finish the exhaust pipe with a similar color tone to those of the other component parts to obtain a consistent look with the surrounding structures.
In general, an exhaust pipe is formed by using carbon steel tubes for machine structural purposes (STKM), stainless steel (SUS), titanium, or the like. In the case of an exhaust pipe composed of STKM, the surface of the exhaust pipe is plated with chromium (Cr) in order to improve the exterior appearance of the exhaust pipe for the aforementioned reason.
However, the exhaust gas traveling through the exhaust pipe, which comes directly from the internal combustion engine, has a high temperature. Therefore, when the exhaust gas passes the exhaust pipe, the exhaust pipe reaches a high temperature, as a result of which the Cr-plated pipe surface may be discolored, oxidized, or otherwise deteriorated. In particular, some regions may suffer from reddish-brown discoloration. Moreover, even in the case of an exhaust pipe which has been formed of SUS or titanium without being Cr-plated, the surface may still be colored reddish-brown due to high temperature, the generated oxide film may peel, and/or change in color tone may result due to oxide film formation.
Thus, due to reddish-brown discoloration or surface deterioration of an exhaust pipe having metallic luster, there occurs a problem in that the exterior appearance of the entire motorcycle design is degraded. This problem is particularly outstanding in the recent years because the performance of internal combustion engines has been improved, resulting in an increased temperature of the exhaust gas.
This problem may be addressed by preventing the temperature of the exhaust pipe surface from becoming too high, by adopting a two-fold or three-fold cylindrical structure for the exhaust pipe. However, even by using a two-fold or three-fold cylindrical structure, the temperature of the exhaust pipe surface will not be adequately reduced, and surface oxidation or deterioration due to heat will not be completely prevented. Moreover, in this case, there is another problem in that the outer dimensions of the exhaust pipe are increased.
It might be possible to prevent discoloration or deterioration of the exhaust pipe surface by covering the exhaust pipe with a cowl or a protector so that the exhaust pipe will not show on the exterior. However, in this case, the exhaust pipe does not contribute to the entire motorcycle design, thus making it difficult to pursue the characteristic beauty of a motorcycle.
In order to solve the above-described problems, the inventor has proposed forming a silicon oxide film on an exhaust pipe of an internal combustion engine in order to prevent discoloration of the exhaust pipe (Japanese Laid-Open Patent Publication No. 2002-332838). As disclosed in Japanese Laid-Open Patent Publication No. 2002-332838, by forming a silicon oxide film having a thickness of about 0.05 μm or more on an exhaust pipe through sol-gel processing, it becomes possible to obtain an exhaust pipe which is unlikely to suffer from discoloration even when heated to a temperature of about 400° C.
However, it has been found that an exhaust pipe which is obtained by the technique disclosed in Japanese Laid-Open Patent Publication No. 2002-332838 may suffer from drastic surface discolorations due to exposure to a temperature exceeding 400° C. It has also been found that the silicon oxide film which is formed according to Japanese Laid-Open Patent Publication No. 2002-332838 is relatively soft, and thus does not have sufficient strength against impact. Therefore, discoloration may not be sufficiently prevented in the case where an exhaust pipe according to Japanese Laid-Open Patent Publication No. 2002-332838 is used for a high-performance internal combustion engine in which the exhaust gas will reach a high temperature as mentioned above. Moreover, in the case of a motorcycle, the exposed exhaust pipe may be bombarded with stones and sand during travel, and discoloration is likely to occur beginning from the marks, scrapes, indents and other deformations left on the surface of the exhaust pipe.
Furthermore, it has been found that interference fringes are likely to occur with a silicon oxide film which has been obtained through sol-gel processing, and that most conspicuous interference fringes appear with discoloration responsive to a temperature above 400° C.